QUEUE(3)                  OpenBSD Programmer's Manual                 QUEUE(3)

NAME
     SLIST_ENTRY, SLIST_HEAD, SLIST_HEAD_INITIALIZER, SLIST_FIRST, SLIST_NEXT,
     SLIST_END, SLIST_EMPTY, SLIST_FOREACH, SLIST_INIT, SLIST_INSERT_AFTER,
     SLIST_INSERT_HEAD, SLIST_REMOVE_HEAD, LIST_ENTRY, LIST_HEAD,
     LIST_HEAD_INITIALIZER, LIST_FIRST, LIST_NEXT, LIST_END, LIST_EMPTY,
     LIST_FOREACH, LIST_INIT, LIST_INSERT_AFTER, LIST_INSERT_BEFORE,
     LIST_INSERT_HEAD, LIST_REMOVE, SIMPLEQ_ENTRY, SIMPLEQ_HEAD,
     SIMPLEQ_HEAD_INITIALIZER, SIMPLEQ_FIRST, SIMPLEQ_NEXT, SIMPLEQ_END,
     SIMPLEQ_EMPTY, SIMPLEQ_FOREACH, SIMPLEQ_INIT, SIMPLEQ_INSERT_HEAD,
     SIMPLEQ_INSERT_TAIL, SIMPLEQ_INSERT_AFTER, SIMPLEQ_REMOVE_HEAD,
     TAILQ_ENTRY, TAILQ_HEAD, TAILQ_HEAD_INITIALIZER, TAILQ_FIRST, TAILQ_NEXT,
     TAILQ_END, TAILQ_LAST, TAILQ_PREV, TAILQ_EMPTY, TAILQ_FOREACH,
     TAILQ_INIT, TAILQ_INSERT_AFTER, TAILQ_INSERT_BEFORE, TAILQ_INSERT_HEAD,
     TAILQ_INSERT_TAIL, TAILQ_REMOVE, CIRCLEQ_ENTRY, CIRCLEQ_HEAD,
     CIRCLEQ_HEAD_INITIALIZER, CIRCLEQ_FIRST, CIRCLEQ_LAST, CIRCLEQ_END,
     CIRCLEQ_NEXT, CIRCLEQ_PREV, CIRCLEQ_EMPTY, CIRCLEQ_FOREACH, CIRCLEQ_INIT,
     CIRCLEQ_INSERT_AFTER, CIRCLEQ_INSERT_BEFORE, CIRCLEQ_INSERT_HEAD,
     CIRCLEQ_INSERT_TAIL, CIRCLEQ_REMOVE - implementations of singly-linked
     lists, doubly-linked lists, simple queues, tail queues, and circular
     queues



SYNOPSIS
     #include <sys/queue.h>


     SLIST_ENTRY(TYPE);

     SLIST_HEAD(HEADNAME, TYPE);

     SLIST_HEAD_INITIALIZER(SLIST_HEAD head);

     struct TYPE *
     SLIST_FIRST(SLIST_HEAD *head);

     struct TYPE *
     SLIST_NEXT(struct TYPE *listelm, SLIST_ENTRY NAME);

     struct TYPE *
     SLIST_END(SLIST_HEAD *head);

     bool
     SLIST_EMPTY(SLIST_HEAD *head);

     SLIST_FOREACH(VARNAME, SLIST_HEAD *head, SLIST_ENTRY NAME);

     void
     SLIST_INIT(SLIST_HEAD *head);

     void
     SLIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm,
             SLIST_ENTRY NAME);

     void
     SLIST_INSERT_HEAD(SLIST_HEAD *head, struct TYPE *elm, SLIST_ENTRY NAME);

     void
     SLIST_REMOVE_HEAD(SLIST_HEAD *head, SLIST_ENTRY NAME);


     LIST_ENTRY(TYPE);

     LIST_HEAD(HEADNAME, TYPE);

     LIST_HEAD_INITIALIZER(LIST_HEAD head);

     struct TYPE *
     LIST_FIRST(LIST_HEAD *head);

     struct TYPE *
     LIST_NEXT(struct TYPE *listelm, LIST_ENTRY NAME);

     struct TYPE *
     LIST_END(LIST_HEAD *head);

     bool
     LIST_EMPTY(LIST_HEAD *head);

     LIST_FOREACH(VARNAME, LIST_HEAD *head, LIST_ENTRY NAME);

     void
     LIST_INIT(LIST_HEAD *head);

     void
     LIST_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm,
             LIST_ENTRY NAME);

     void
     LIST_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm,
             LIST_ENTRY NAME);

     void
     LIST_INSERT_HEAD(LIST_HEAD *head, struct TYPE *elm, LIST_ENTRY NAME);

     void
     LIST_REMOVE(struct TYPE *elm, LIST_ENTRY NAME);


     SIMPLEQ_ENTRY(TYPE);

     SIMPLEQ_HEAD(HEADNAME, TYPE);

     SIMPLEQ_HEAD_INITIALIZER(SIMPLEQ_HEAD head);

     struct TYPE *
     SIMPLEQ_FIRST(SIMPLEQ_HEAD *head);

     struct TYPE *
     SIMPLEQ_NEXT(struct TYPE *listelm, SIMPLEQ_ENTRY NAME);

     struct TYPE *
     SIMPLEQ_END(SIMPLEQ_HEAD *head);

     void
     SIMPLEQ_INIT(SIMPLEQ_HEAD *head);

     void
     SIMPLEQ_INSERT_HEAD(SIMPLEQ_HEAD *head, struct TYPE *elm,
             SIMPLEQ_ENTRY NAME);

     void
     SIMPLEQ_INSERT_TAIL(SIMPLEQ_HEAD *head, struct TYPE *elm,
             SIMPLEQ_ENTRY NAME);

     void
     SIMPLEQ_INSERT_AFTER(struct TYPE *listelm, struct TYPE *elm,
             SIMPLEQ_ENTRY NAME);

     void
     SIMPLEQ_REMOVE_HEAD(SIMPLEQ_HEAD *head, struct TYPE *elm,
             SIMPLEQ_ENTRY NAME);


     TAILQ_ENTRY(TYPE);

     TAILQ_HEAD(HEADNAME, TYPE);

     TAILQ_HEAD_INITIALIZER(TAILQ_HEAD head);

     struct TYPE *
     TAILQ_FIRST(TAILQ_HEAD *head);

     struct TYPE *
     TAILQ_NEXT(struct TYPE *listelm, TAILQ_ENTRY NAME);

     struct TYPE *
     TAILQ_END(TAILQ_HEAD *head);

     struct TYPE *
     TAILQ_LAST(TAILQ_HEAD *head, HEADNAME NAME);

     TAILQ_PREV(TAILQ_HEAD *head, HEADNAME NAME);

     void
     TAILQ_INIT(TAILQ_HEAD *head);

     void
     TAILQ_INSERT_AFTER(TAILQ_HEAD *head, struct TYPE *listelm,
             struct TYPE *elm, TAILQ_ENTRY NAME);

     void
     TAILQ_INSERT_BEFORE(struct TYPE *listelm, struct TYPE *elm,
             TAILQ_ENTRY NAME);

     void
     TAILQ_INSERT_HEAD(TAILQ_HEAD *head, struct TYPE *elm, TAILQ_ENTRY NAME);

     void
     TAILQ_INSERT_TAIL(TAILQ_HEAD *head, struct TYPE *elm, TAILQ_ENTRY NAME);

     void
     TAILQ_REMOVE(TAILQ_HEAD *head, struct TYPE *elm, TAILQ_ENTRY NAME);


     CIRCLEQ_ENTRY(TYPE);

     CIRCLEQ_HEAD(HEADNAME, TYPE);

     CIRCLEQ_HEAD_INITIALIZER(CIRCLEQ_HEAD head);

     struct TYPE *
     CIRCLEQ_FIRST(CIRCLEQ_HEAD *head);

     struct TYPE *
     CIRCLEQ_LAST(CIRCLEQ_HEAD *head);

     struct TYPE *
     CIRCLEQ_END(CIRCLEQ_HEAD *head);

     struct TYPE *
     CIRCLEQ_NEXT(struct TYPE *listelm, CIRCLEQ_ENTRY NAME);

     struct TYPE *
     CIRCLEQ_PREV(struct TYPE *listelm, CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_INIT(CIRCLEQ_HEAD *head);

     void
     CIRCLEQ_INSERT_AFTER(CIRCLEQ_HEAD *head, struct TYPE *listelm,
             struct TYPE *elm, CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_INSERT_BEFORE(CIRCLEQ_HEAD *head, struct TYPE *listelm,
             struct TYPE *elm, CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_INSERT_HEAD(CIRCLEQ_HEAD *head, struct TYPE *elm,
             CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_INSERT_TAIL(CIRCLEQ_HEAD *head, struct TYPE *elm,
             CIRCLEQ_ENTRY NAME);

     void
     CIRCLEQ_REMOVE(CIRCLEQ_HEAD *head, struct TYPE *elm, CIRCLEQ_ENTRY NAME);

DESCRIPTION
     These macros define and operate on five types of data structures: singly-
     linked lists, simple queues, lists, tail queues, and circular queues.
     All five structures support the following functionality:

           1.   Insertion of a new entry at the head of the list.
           2.   Insertion of a new entry after any element in the list.
           3.   Removal of an entry from the head of the list.
           4.   Forward traversal through the list.

     Singly-linked lists are the simplest of the five data structures and sup-
     port only the above functionality.  Singly-linked lists are ideal for ap-
     plications with large datasets and few or no removals, or for implement-
     ing a LIFO queue.

     Simple queues add the following functionality:

           1.   Entries can be added at the end of a list.

     However:

           1.   All list insertions must specify the head of the list.
           2.   Each head entry requires two pointers rather than one.
           3.   Code size is about 15% greater and operations run about 20%
                slower than singly-linked lists.

     Simple queues are ideal for applications with large datasets and few or
     no removals, or for implementing a FIFO queue.

     All doubly linked types of data structures (lists, tail queues, and cir-
     cle queues) additionally allow:

           1.   Insertion of a new entry before any element in the list.
           2.   Removal of any entry in the list.

     However:

           1.   Each elements requires two pointers rather than one.
           2.   Code size and execution time of operations (except for re-
                moval) is about twice that of the singly-linked data-struc-
                tures.

     Lists are the simplest of the doubly linked data structures and support
     only the above functionality over singly-linked lists.

     Tail queues add the following functionality:

           1.   Entries can be added at the end of a list.
           2.   They may be traversed backwards, at a cost.

     However:

           1.   All list insertions and removals must specify the head of the
                list.
           2.   Each head entry requires two pointers rather than one.
           3.   Code size is about 15% greater and operations run about 20%
                slower than singly-linked lists.

     Circular queues add the following functionality:

           1.   Entries can be added at the end of a list.
           2.   They may be traversed backwards, from tail to head.

     However:

           1.   All list insertions and removals must specify the head of the
                list.
           2.   Each head entry requires two pointers rather than one.
           3.   The termination condition for traversal is more complex.
           4.   Code size is about 40% greater and operations run about 45%
                slower than lists.

     In the macro definitions, TYPE is the name tag of a user defined struc-
     ture, that must contain a field of type SLIST_ENTRY, LIST_ENTRY,
     SIMPLEQ_ENTRY, TAILQ_ENTRY, or CIRCLEQ_ENTRY, named NAME. The argument
     HEADNAME is the name tag of a user defined structure that must be de-
     clared using the macros SLIST_HEAD(), LIST_HEAD(), SIMPLEQ_HEAD(),
     TAILQ_HEAD(), or CIRCLEQ_HEAD().  See the examples below for further ex-
     planation of how these macros are used.

SINGLY_LINKED LISTS
     A singly-linked list is headed by a structure defined by the SLIST_HEAD()
     macro.  This structure contains a single pointer to the first element on
     the list.  The elements are singly linked for minimum space and pointer
     manipulation overhead at the expense of O(n) removal for arbitrary ele-
     ments.  New elements can be added to the list after an existing element
     or at the head of the list.  A SLIST_HEAD structure is declared as fol-
     lows:

           SLIST_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name of the structure to be defined, and struct
     TYPE is the type of the elements to be linked into the list.  A pointer
     to the head of the list can later be declared as:

           struct HEADNAME *headp;

     (The names head and headp are user selectable.)

     The HEADNAME facility is often not used, leading to the following bizarre
     code:

           SLIST_HEAD(, TYPE) head, *headp;

     The SLIST_ENTRY() macro declares a structure that connects the elements
     in the list.

     The SLIST_INIT() macro initializes the list referenced by head.

     The list can also be initialized statically by using the
     SLIST_HEAD_INITIALIZER() macro like this:

           SLIST_HEAD(HEADNAME, TYPE) head = SLIST_HEAD_INITIALIZER(head);

     The SLIST_INSERT_HEAD() macro inserts the new element elm at the head of
     the list.

     The SLIST_INSERT_AFTER() macro inserts the new element elm after the ele-
     ment listelm.

     The SLIST_REMOVE_HEAD() macro removes the first element of the list
     pointed by head.

     The SLIST_FIRST(), and SLIST_NEXT() macros can be used to traverse the
     list:

           for (np = SLIST_FIRST(&head); np != NULL; np = SLIST_NEXT(np, NAME))
     Or, for simplicity, one can use the SLIST_FOREACH() macro:

           SLIST_FOREACH(np, head, NAME)

     The SLIST_EMPTY() macro should be used to check whether a simple list is
     empty.

LISTS
     A list is headed by a structure defined by the LIST_HEAD() macro.  This
     structure contains a single pointer to the first element on the list.
     The elements are doubly linked so that an arbitrary element can be re-
     moved without traversing the list.  New elements can be added to the list
     after an existing element, before an existing element, or at the head of
     the list.  A LIST_HEAD structure is declared as follows:

           LIST_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name of the structure to be defined, and struct
     TYPE is the type of the elements to be linked into the list.  A pointer
     to the head of the list can later be declared as:

           struct HEADNAME *headp;

     (The names head and headp are user selectable.)

     The HEADNAME facility is often not used, leading to the following bizarre
     code:

           LIST_HEAD(, TYPE) head, *headp;

     The LIST_ENTRY() macro declares a structure that connects the elements in
     the list.

     The LIST_INIT() macro initializes the list referenced by head.

     The list can also be initialized statically by using the
     LIST_HEAD_INITIALIZER() macro like this:

           LIST_HEAD(HEADNAME, TYPE) head = LIST_HEAD_INITIALIZER(head);

     The LIST_INSERT_HEAD() macro inserts the new element elm at the head of
     the list.

     The LIST_INSERT_AFTER() macro inserts the new element elm after the ele-
     ment listelm.

     The LIST_INSERT_BEFORE() macro inserts the new element elm before the el-
     ement listelm.

     The LIST_REMOVE() macro removes the element elm from the list.
     The LIST_FIRST(), and LIST_NEXT() macros can be used to traverse the
     list:

           for (np = LIST_FIRST(&head); np != NULL; np = LIST_NEXT(np, NAME))
     Or, for simplicity, one can use the LIST_FOREACH() macro:

           LIST_FOREACH(np, head, NAME)

     The LIST_EMPTY() macro should be used to check whether a list is empty.

LIST EXAMPLE
     LIST_HEAD(listhead, entry) head;
     struct listhead *headp;         /* List head. */
     struct entry {
             ...
             LIST_ENTRY(entry) entries;      /* List. */
             ...
     } *n1, *n2, *np;

     LIST_INIT(&head);                       /* Initialize the list. */

     n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
     LIST_INSERT_HEAD(&head, n1, entries);

     n2 = malloc(sizeof(struct entry));      /* Insert after. */
     LIST_INSERT_AFTER(n1, n2, entries);

     n2 = malloc(sizeof(struct entry));      /* Insert before. */
     LIST_INSERT_BEFORE(n1, n2, entries);
                                             /* Forward traversal. */
     for (np = head.lh_first; np != NULL; np = np->entries.le_next)
             np-> ...

     while (head.lh_first != NULL)           /* Delete. */
             LIST_REMOVE(head.lh_first, entries);

SIMPLE QUEUES
     A simple queue is headed by a structure defined by the SIMPLEQ_HEAD()
     macro.  This structure contains a pair of pointers, one to the first ele-
     ment in the simple queue and the other to the last element in the simple
     queue.  The elements are singly linked.  New elements can be added to the
     queue after an existing element, at the head of the queue or at the tail
     of the queue.  A SIMPLEQ_HEAD structure is declared as follows:

           SIMPLEQ_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name of the structure to be defined, and struct
     TYPE is the type of the elements to be linked into the queue.  A pointer
     to the head of the queue can later be declared as:

           struct HEADNAME *headp;

     (The names head and headp are user selectable.)

     The SIMPLEQ_ENTRY() macro declares a structure that connects the elements
     in the queue.

     The SIMPLEQ_INIT() macro initializes the queue referenced by head.

     The queue can also be initialized statically by using the
     SIMPLEQ_HEAD_INITIALIZER() macro like this:

           SIMPLEQ_HEAD(HEADNAME, TYPE) head = SIMPLEQ_HEAD_INITIALIZER(head);

     The SIMPLEQ_INSERT_HEAD() macro inserts the new element elm at the head
     of the queue.
     The SIMPLEQ_INSERT_TAIL() macro inserts the new element elm at the end of
     the queue.

     The SIMPLEQ_INSERT_AFTER() macro inserts the new element elm after the
     element listelm.

     The SIMPLEQ_REMOVE_HEAD() macro removes the first element from the queue.

     The SIMPLEQ_FIRST(), and SIMPLEQ_NEXT() macros can be used to traverse
     the queue.  The SIMPLEQ_FOREACH() is used for queue traversal

           SIMPLEQ_FOREACH(np, head, NAME)

     The SIMPLEQ_EMPTY() macro should be used to check whether a list is emp-
     ty.

SIMPLE QUEUE EXAMPLE
     SIMPLEQ_HEAD(listhead, entry) head = SIMPLEQ_HEAD_INITIALIZER(head);
     struct entry {
             ...
             SIMPLEQ_ENTRY(entry) entries;   /* List. */
             ...
     } *n1, *n2, *np;

     n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
     SIMPLEQ_INSERT_HEAD(&head, n1, entries);

     n2 = malloc(sizeof(struct entry));      /* Insert after. */
     SIMPLEQ_INSERT_AFTER(n1, n2, entries);

     n2 = malloc(sizeof(struct entry));      /* Insert at the tail. */
     SIMPLEQ_INSERT_TAIL(&head, n1, entries);
                                             /* Forward traversal. */
     for (np = SIMPLEQ_FIRST(&head); np != NULL; np = SIMPLEQ_NEXT(np, entries))
             np-> ...
                                             /* Delete. */
     while (SIMPLEQ_FIRST(&head) != NULL)
             SIMPLEQ_REMOVE_HEAD(&head, n1, entries);

TAIL QUEUES
     A tail queue is headed by a structure defined by the TAILQ_HEAD() macro.
     This structure contains a pair of pointers, one to the first element in
     the tail queue and the other to the last element in the tail queue.  The
     elements are doubly linked so that an arbitrary element can be removed
     without traversing the tail queue.  New elements can be added to the
     queue after an existing element, before an existing element, at the head
     of the queue, or at the end the queue.  A TAILQ_HEAD structure is de-
     clared as follows:

           TAILQ_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name of the structure to be defined, and struct
     TYPE is the type of the elements to be linked into the tail queue.  A
     pointer to the head of the tail queue can later be declared as:

           struct HEADNAME *headp;

     (The names head and headp are user selectable.)

     The TAILQ_ENTRY() macro declares a structure that connects the elements
     in the tail queue.

     The TAILQ_INIT() macro initializes the tail queue referenced by head.

     The tail queue can also be initialized statically by using the
     TAILQ_HEAD_INITIALIZER() macro.
     The TAILQ_INSERT_HEAD() macro inserts the new element elm at the head of
     the tail queue.

     The TAILQ_INSERT_TAIL() macro inserts the new element elm at the end of
     the tail queue.

     The TAILQ_INSERT_AFTER() macro inserts the new element elm after the ele-
     ment listelm.

     The TAILQ_INSERT_BEFORE() macro inserts the new element elm before the
     element listelm.

     The TAILQ_REMOVE() macro removes the element elm from the tail queue.

     The TAIL_FIRST(), TAILQ_NEXT(), TAILQ_LAST() and TAILQ_PREV() macros can
     be used to traverse a tail queue.  The TAILQ_FOREACH() is used for tail
     queue traversal

           TAILQ_FOREACH(np, head, NAME)

     The TAILQ_EMPTY() macro should be used to check whether a tail queue is
     empty.

TAIL QUEUE EXAMPLE
     TAILQ_HEAD(tailhead, entry) head;
     struct tailhead *headp;         /* Tail queue head. */
     struct entry {
             ...
             TAILQ_ENTRY(entry) entries;     /* Tail queue. */
             ...
     } *n1, *n2, *np;

     TAILQ_INIT(&head);                      /* Initialize the queue. */

     n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
     TAILQ_INSERT_HEAD(&head, n1, entries);

     n1 = malloc(sizeof(struct entry));      /* Insert at the tail. */
     TAILQ_INSERT_TAIL(&head, n1, entries);

     n2 = malloc(sizeof(struct entry));      /* Insert after. */
     TAILQ_INSERT_AFTER(&head, n1, n2, entries);

     n2 = malloc(sizeof(struct entry));      /* Insert before. */
     TAILQ_INSERT_BEFORE(n1, n2, entries);
                                             /* Forward traversal. */
     for (np = head.tqh_first; np != NULL; np = np->entries.tqe_next)
             np-> ...
                                             /* Delete. */
     while (head.tqh_first != NULL)
             TAILQ_REMOVE(&head, head.tqh_first, entries);

CIRCULAR QUEUES
     A circular queue is headed by a structure defined by the CIRCLEQ_HEAD()
     macro.  This structure contains a pair of pointers, one to the first ele-
     ment in the circular queue and the other to the last element in the cir-
     cular queue.  The elements are doubly linked so that an arbitrary element
     can be removed without traversing the queue.  New elements can be added
     to the queue after an existing element, before an existing element, at
     the head of the queue, or at the end of the queue.  A CIRCLEQ_HEAD struc-
     ture is declared as follows:

           CIRCLEQ_HEAD(HEADNAME, TYPE) head;

     where HEADNAME is the name of the structure to be defined, and struct
     TYPE is the type of the elements to be linked into the circular queue.  A
     pointer to the head of the circular queue can later be declared as:

           struct HEADNAME *headp;

     (The names head and headp are user selectable.)

     The CIRCLEQ_ENTRY() macro declares a structure that connects the elements
     in the circular queue.

     The CIRCLEQ_INIT() macro initializes the circular queue referenced by
     head.

     The circular queue can also be initialized statically by using the
     CIRCLEQ_HEAD_INITIALIZER() macro.

     The CIRCLEQ_INSERT_HEAD() macro inserts the new element elm at the head
     of the circular queue.

     The CIRCLEQ_INSERT_TAIL() macro inserts the new element elm at the end of
     the circular queue.

     The CIRCLEQ_INSERT_AFTER() macro inserts the new element elm after the
     element listelm.

     The CIRCLEQ_INSERT_BEFORE() macro inserts the new element elm before the
     element listelm.

     The CIRCLEQ_REMOVE() macro removes the element elm from the circular
     queue.

     The CIRCLEQ_FIRST(), CIRCLEQ_LAST(), CIRCLEQ_END(), CIRCLEQ_NEXT() and
     CIRCLEQ_PREV() macros can be used to traverse a circular queue.  The
     CIRCLEQ_FOREACH() is used for circular queue forward traversal

           CIRCLEQ_FOREACH(np, head, NAME)

     The CIRCLEQ_EMPTY() macro should be used to check whether a circular
     queue is empty.

CIRCULAR QUEUE EXAMPLE
     CIRCLEQ_HEAD(circleq, entry) head;
     struct circleq *headp;                  /* Circular queue head. */
     struct entry {
             ...
             CIRCLEQ_ENTRY entries;          /* Circular queue. */
             ...
     } *n1, *n2, *np;

     CIRCLEQ_INIT(&head);                    /* Initialize the circular queue. */

     n1 = malloc(sizeof(struct entry));      /* Insert at the head. */
     CIRCLEQ_INSERT_HEAD(&head, n1, entries);

     n1 = malloc(sizeof(struct entry));      /* Insert at the tail. */
     CIRCLEQ_INSERT_TAIL(&head, n1, entries);

     n2 = malloc(sizeof(struct entry));      /* Insert after. */
     CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);

     n2 = malloc(sizeof(struct entry));      /* Insert before. */
     CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
                                             /* Forward traversal. */
     for (np = CIRCLEQ_FIRST(&head); np != CIRCLEQ_END(&head);
         np = CIRCLEQ_NEXT(np, entries))
             np-> ...
                                             /* Reverse traversal. */
     for (np = CIRCLEQ_LAST(&head); np != CIRCLEQ_END(&head);
         np = CIRCLEQ_PREV(np, entries))
             np-> ...
                                             /* Delete. */
     while (CIRCLEQ_FIRST(&head) != CIRCLEQ_END(&head))
             CIRCLEQ_REMOVE(&head, CIRCLEQ_FIRST(&head), entries);

NOTES
     The SLIST_END(), LIST_END(), SIMPLEQ_END() and TAILQ_END() macros are
     provided for symetry with CIRCLEQ_END().  They expand to NULL and don't
     serve any useful purpose.

     Trying to free a list in the following way is a common error:

           LIST_FOREACH(var, head, entry)
                   free(var);
           free(head);
     Since var is free'd, the FOREACH() macro refers to a pointer that may
     have been reallocated already.  Proper code needs a second variable.

           for (var = LIST_FIRST(head); var != LIST_END(head); var = nxt) {
                   nxt = LIST_NEXT(var);
                   free(var);
           }
           LIST_INIT(head);        /* to put the list back in order */

HISTORY
     The queue functions first appeared in 4.4BSD.

OpenBSD 2.6                    December 13, 1993                            11